A delayed coking unit is extensively used for producing cokes from petroleum or coal heavy oils. The delayed coking unit, in general, basically comprises a heating furnace of feed material heavy oil and two or more coking drums which have been provided in parallel to each other and connected in series in this sequence, wherein the delayed coking process is carried out by feeding the heavy oil which has been heated to a thermal cracking temperature of 450.degree.-500.degree. C. into one of the coking drums by a pump, causing it to reside for a relatively long period, and accumulating coke produced by the cracking of the heavy oil within said coking drums, while oils resulting from the cracking are distilled out from the top of the drums.
However, the delayed coking process has a problem in that coke of high quality cannot be obtained from feed materials other than heavy oils of a definite quality, whereby a number of techniques for producing coke of good quality from a variety of feed material heavy oils have been proposed. These techniques include, for example: a process of previously subjecting a petroleum heavy oil to thermal cracking to an appropriate extent by means of a thermal cracking unit and supplying the resulting tar as a starting material or a part of the starting material to a coking drum (Japanese Patent Publication No. 33901/74, etc.); a process of using a clarified oil supplied from a catalytic cracking unit as a blended starting material oil (Japanese Patent Publication No. 18176/60, etc.); and a process for producing coke by using two coking drums arranged in series wherein coke of inferior quality is produced from the oil components of the starting material oil in earlier coking stages in the drum of the first stage, and coke of high quality is produced from the remaining oil components in the drum of the second stage (Japanese Patent Application Laid-Open No. 89902/73). The first and second processes described above are processes belonging to the province of the selection or pretreatment of the feed material oil, and the last one is a process which is primarily aimed at the removal of inferior components in the feed material oil, either of which processes is in general a technique characterized by controlling the composition of the feed material oil.
It may be said that this invention in a broad sense relates to the preparation technique of the feed material oil. An object of this invention is to provide a technique of producing coke of high quality from feed material oils having a variety of qualities by relatively simple alternation of a unit and the enlargement of the operation range in the reaction condition.
In the conventional delayed coking unit which has hitherto been used, a fluid which has been heated to a prescribed temperature in a heating furnace is merely transferred to a coking drum which is kept at a certain level of temperature. Further, the coking drum requires a huge volume in order to promote the desired thermal cracking and coking, and it is impossible in fact to heat such a huge coking drum and to control its temperature. Therefore, the coking drum is merely maintained at a natural temperature at which a balance is attained with the heat brought into the drum by the fluid which has been heated in the heating furnace. In other words, the temperature of the coking drum depends on only the temperature at the outlet of the heating furnace. The operation variable factors include additionally the flow rate and pressure of the feed material oil, but the operation ranges of these factors in actual industrial production are limited to such narrow ranges by design restrictions that these factors cannot be freely varied to improve effectively the quality of the coke product.
Meanwhile, the present inventors, as a result of engagement and researches in the production of coke over a long period of years, have found that the quality of the coke article is influenced very seriously not only by the temperature condition in the coking drum but also by the coking or thermal cracking condition of the feed material oil in the earlier stages, particularly the thermal cracking condition in the pathway from the heating furnace to the coking drum. That is to say, it has been found possible to control the quality of coke not by controlling the reaction condition in the coking drum which is difficult to control but by controlling the preliminary thermal cracking condition. Further, the preliminary thermal cracking process is completed in relatively short time, so that its control is distinguishedly simple in comparison with the control in the huge coking drum.